The present invention relates to holographic optical devices, and particularly to devices which include a plurality of holographic optical elements (HOEs) carried by a common light-transmissive substrate.
One of the important applications for HOEs is in visor displays, in which a HOE serves as an imaging lens and a combiner where a two-dimensional, quasi-monochromatic display is imaged to infinity and reflected into the eye of an observer. The display can be obtained directly, either from a cathode ray tube (CRT) or a liquid crystal display (LCD); or indirectly, by means of a relay lens or an optical fiber bundle. Typically, the display is comprised of an array of points, the geometry of which differs from that used for recording the HOE. As a result, the imaged array contains aberrations that decrease the image quality. Usually, the amount of aberrations increases as the offset angle for recording the HOE becomes larger. Another problem, which is usually common to all types of holographic optical elements, is their relatively high chromatic dispersion. This is a major drawback in applications where the light source is a CRT which is not purely monochromatic.
Recently, several new designs were proposed for improving the performance of HOES. These designs, which usually deal with single HOEs, compensate for the geometric and chromatic aberrations by using non spherical waves rather than simple spherical waves for recording however, they do not overcome the chromatic dispersion problem. Moreover, with these designs, the overall optical systems which include these HOEs are usually very complicated and difficult to manufacture. Furthermore, the eye-motion-box of the optical viewing angles resulting from these designs is usually very small typically less than 10 mm. Hence, the performance of the optical system is very sensitive, even to small movements of the visor relative to the eye of the viewer.
The present invention may be used for designing and recording HOEs for visor displays in which both the aberrations and chromatic dispersions are minimized. In addition, the eye-motion-box of the optical system is relatively large and can therefore accommodate large movements of the visor. The HOE of the invention is particularly advantageous because it is very compact and can be readily incorporated even into optical systems that have specialized configurations.
The present invention also enables the construction of improved holographic displays. Since its inception three decades ago, there has been significant progress in the area of display holography. Indeed, it has become so popular that it now plays an important role in advertising, packaging and even in art. Yet, the current form of display holograms has some severe drawbacks the necessity to use a readout light source that must be located at some distance from the hologram, in order to illuminate its entire surface, makes holographic display systems bulky, space-consuming and sometimes inconvenient to use. Another drawback is that the transmitted part of the readout wave which is not diffracted by the holograms usually bothers the observer.
Another important application of the present invention is in providing a compact holographic display which overcomes the above drawbacks. The holographic display of the invention can be illuminated with a compact light source which is attached to the holographic substrate, normal to the smallest HOE recorded on the substrate. The chromatic dispersion of this display is relatively small and the light source can have a wide spectrum, even like a conventional white-light source. In addition, the area of the display can be much larger than the area which is actually illuminated by the light source.
A further application of the invention provides a compact beam expander. Beam expanders for magnifying a narrow, collimated beam into a beam of larger diameter typically comprise a telescopic assembly of two lenses along a common axis with a common focal point when used with polychromatic light, e.g., white light, refractive lenses are generally needed. However, with monochromatic light, such as from lasers or other monochromatic light sources, it may be advantageous to exploit holographic lenses. The present invention provides a beam expander which can be used with monochromatic light as well as with polychromatic light.
It is therefore a broad object of the present invention to provide compact holographic optical devices in which the readout light source must not be located at some distance from the hologram in order to illuminate the entire surface, and the light source can be constituted by a white-light source.
Therefore, according to the present invention there is provided a holographic optical device comprising a light-transmissive substrate; a first holographic optical element carried by said substrate; at least one second holographic optical element carried by said substrate laterally of said first holographic optical element, and at least one third holographic optical element carried by said substrate laterally displaced from said first and second holographic optical elements; characterized in that the center of at least one of said first, second or third holographic optical elements is located outside a single, straight line, and wherein the lateral dimensions of the area of display of said third holographic optical element are larger than the lateral dimensions of the area of display of said first holographic optical element.